Bioisosteric replacement of anilide with benzoxazole: Potent and orally bioavailable antagonists of VLA-4

Article abstract of DOI:10.1016/j.bmcl.2004.01.098

We have designed and synthesized a series of heterocyclic bioisosteres for an anilide based on molecular modeling. Excellent potency was retained in the benzoxazole and the benzimidazole derivatives, where a hydrogen bond acceptor is appropriately positioned to mimic the amide bond oxygen. The deletion of the hydrogen bond donor (N-H) led to improved lipophilicity and bioavailability. In the process, 9a was identified as a potent, specific, and bioavailable VLA-4 antagonist, while 9c was found to be a potent and bioavailable dual antagonist of VLA-4 and α₄β₇.

Full text of DOI:10.1016/j.bmcl.2004.01.098

Bioorganic & Medicinal Chemistry Letters 14 (2004) 2331–2334
Bioisosteric replacement of anilide with benzoxazole: potent and
orally bioavailable antagonists of VLA-4
Linus S. Lin,a,* Thomas J. Lanza, Jr.,a Laurie A. Castonguay,b Theodore Kamenecka,a
Ermenegilda McCauley,c Gail Van Riper,c Linda A. Egger,c Richard A. Mumford,c
Xinchun Tong,a Malcolm MacCoss,a John A. Schmidtc and William K. Hagmanna
aDepartment of Medicinal Chemistry, Merck Research Laboratories, Rahway, NJ 07065, USA
bDepartment of Molecular Systems, Merck Research Laboratories, Rahway, NJ 07065, USA
cDepartment of Inflammation and Rheumatology Research, Merck Research Laboratories, Rahway, NJ 07065, USA
Received 25 November 2003; accepted 29 January 2004
Abstract—We have designed and synthesized a series of heterocyclic bioisosteres for an anilide based on molecular modeling.
Excellent potency was retained in the benzoxazole and the benzimidazole derivatives, where a hydrogen bond acceptor is appro-
priately positioned to mimic the amide bond oxygen. The deletion of the hydrogen bond donor (N–H) led to improved lipophilicity
and bioavailability. In the process, 9a was identified as a potent, specific, and bioavailable VLA-4 antagonist, while 9c was found to
be a potent and bioavailable dual antagonist of VLA-4 and a4b7.
ꢀ 2004 Elsevier Ltd. All rights reserved.
Integrins are a super family of structurally related
heterodimeric glycoproteins, which are involved in cell
adhesion and cell trafficking. The a4b1 integrin (very late
antigen-4, VLA-4) is expressed on many leukocytes
including eosinophils, basophils, and monocytes, but
not on mature neutrophils. Therefore, VLA-4 antago-
nists have potentials for the treatment of allergic dis-
eases such as asthma, and other chronic inflammatory
diseases.1 The a4b7 integrin is found primarily on
mucosal lymphocytes, and blockade of a4b7 may be
beneficial in the treatment of inflammatory bowl disease
or CrohnÕs disease.2 In general, a specific inhibitor of
either a4b1 or a4b7 may be more desirable for safety
concerns, though a dual antagonist may be advanta-
geous to achieve maximum efficacy.3
respectively).5 Both compounds are ca. 100· or more
selective for a4b1 versus a4b7. However, both com-
pounds suffer from poor pharmacokinetic properties.
Specifically, 1 seems to be absorbed but rapidly cleared
in the rat (Clp 97 mL/min/kg, F% 9). On the other hand,
2 is more slowly cleared in vivo but poorly absorbed
(Clp 33 mL/min/kg, F% 0.7). We reasoned that the
hydrophilic nature of the anilide bond may be respon-
sible for the poor bioavailability of 2, and started an
effort to replace the anilide functionality with an
appropriate heterocycle.6
Me
O
H
N
H
N
CO2H
CO2H
N
SO2
N
SO2
O
Cl
Cl
OMe
NH Cl
Previously, we reported a series of N-substituted
dipeptides and found that the N-arylsulfonyl group is
essential for high specificity against a4b1 versus a4b7.4
Further optimization led to the discovery of 1 and 2 as
potent VLA-4 antagonists against both the Mn2þ acti-
vated states (0.07 and 0.27 nM, respectively), and the
unactivated states (Ca2þ/Mg2þ states: 0.27 and 0.31 nM,
Cl
Cl
O
MeO
2
1
Cl
We considered all four possible modes of cyclization for
the heterocycle formation. Each mode would restrict the
two dihedral angles (a and bÞ to a different degree. In
particular, mode a is likely to disrupt the trans-
amide bond configuration. Our goal is to maintain the
* Corresponding author. Tel.: +732-594-8391; fax: +732-594-5966;
e-mail: linus_lin@merck.com
0960-894X/$ - see front matter ꢀ 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.01.098

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L. S. Lin et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2331–2334
preferred orientation of the amide bond and the two aryl
rings. Mechanical calculations were then performed to
help to differentiate modes b, c, and d.
OMe
OMe
O
a,b
c,d
O
N
70%
47%
O
Cl
NH2
OH
a
Cl
3
4
b
NH
NH Cl
α
O
Br
H2N
CO2tBu
c
O
β
Cl
e,f
g,h
d
N
N
A
52%
65%
B
O
Cl
O
Cl
Cl
Cl
The results of the molecular modeling (MacroModel
v5.5) are illustrated in Figure 1. It appears that the
amide bond prefers to be coplanar with the aniline ring
(a ¼ 0ꢁ or 180ꢁ), while the dichlorophenyl group prefers
to be perpendicular to the amide bond (b ¼ 90ꢁ or 270ꢁ),
which constitutes the lowest energy conformation. In
contrast, the torsional strain reaches a maximum of
99 kcal/mol when the amide bond is perpendicular to the
aniline ring (a ¼ 90ꢁ or 270ꢁ), and is coplanar with the
dichlorophenyl ring (b ¼ 0ꢁ or 180ꢁ). These results can
be understood by maximizing the resonance effect
between the amide bond and the aromatic ring while
avoiding steric repulsions between the chlorine atoms
and the amide bond. Clearly, neither mode b nor d
cyclization is desirable in maintaining the preferred
dihedral angle b at 90ꢁ or 270ꢁ, and only mode c is
capable of preserving the preferred conformation of the
aryl anilide. Therefore, mode c cyclization was pursued
in our synthesis.
5
6
H
N
CO2H
N
SO2
O
Cl
N
O
Cl
Cl
Cl
7
Scheme 1. Reagents and conditions: (a) 2,6-Cl2–PhCHO, toluene,
reflux; (b) Pb(OAc)4, AcOH 0 ꢁC to rt, 4 h; (c) LiAlH4, THF, )30 to
0 ꢁC; (d) NBS, Me2S, CH2Cl2, )20 ꢁC to rt; (e) Ph2C@NCH2CO2tBu,
CsOH monohydrate, catalyst, CH2Cl2 (Ref. 8); (f) AcOH/THF/H2O,
1:1:1, rt, 30 min; (g) RCO2H, PyBop, iPr2NEt, 0 ꢁC to rt; (f) TFA,
CH2Cl2, rt.
postulated that the loss of potency was due to less
optimal binding to the oxazole versus the original
amide. Assuming that the amide bond oxygen is inter-
acting with the VLA-4 receptor through hydrogen-
bonding interactions, a better hydrogen bond acceptor
than the oxygen of benzoxazole 7 may be more desir-
able. To test this hypothesis, the regioisomeric benzox-
azole and the benzimidazole derivative were synthesized
(Scheme 2). In both cases, moderate to significant
improvement in potency against activated and unacti-
vated states of VLA-4 were observed (Table 1: 7 vs 9a
and 14a: 1.7 nM, vs 0.74 nM and 0.54 nM (Mn2þ states);
33 nM vs 3.6 nM, and 1.8 nM (Ca2þ/Mg2þ states)),
which is consistent with the improved hydrogen bonding
capability of the oxygen of benzoxazole 9 and the
nitrogen of benzimidazole 14. Three other compounds
(9b,c, and 14b) were also prepared based on literature
precedents,9 and they were also found to be potent
VLA-4 antagonists. Significant activities against a4b7
were also observed for these three compounds, which is
consistent with previous findings that arylsulfonyl group
is essential for a4b1 selectivity.4
Synthesis of the benzoxazole derivatives followed the
procedures illustrated in Scheme 1. The key steps are
benzoxazole formation7 and cinchona alkaloid-cata-
lyzed alkylation of glycinate developed by Corey et al.8
Compound 7 was studied in the binding assays and was
found to be significantly less active than 2 (1.7 nM vs
0.27 nM) against VLA-4. Since little conformational
change from anilide to benzoxazole was expected, it was
The pharmacokinetic properties of these new com-
pounds were evaluated in the rat (Table 1).10 The bio-
availability of benzoxazole 9a improved significantly in
comparison with 2 (F%: 15 vs 0.7), suggesting that the
polar anilide bond may be responsible for the poor
bioavailability of 2. This is also consistent with the
observed decrease in bioavailability of 14a, which is less
lipophilic than both 2 and 9a (log D for 2, 9a, and 14a:
Figure 1. Torsional strain of fragment B as a function of dihedral
angle a and b (MacroModel v5.5). The energy level is color coded with
purple being the lowest at 219 kcal/mol and red being the highest at
318 kcal/mol.

L. S. Lin et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2331–2334
2333
H
N
good pharmacokinetic properties in the rat. These
compounds have much-reduced molecular weight and
are very attractive candidates for further evaluations.
R
CO2H
OMe
O
same as
O
Scheme 1
O
OH
In summary, we have designed and synthesized a num-
ber of heterocyclic mimetics for the anilide bond based
on molecular modeling. Excellent potency was retained
by positioning a suitable hydrogen bond acceptor to
mimic the amide bond oxygen. The removal of the
hydrogen bond donor (N–H) led to improved lipophi-
licity and bioavailability. In the process, 9a was identi-
fied as a potent, specific, and bioavailable VLA-4
antagonist, while 9c was found to be a potent and bio-
available dual antagonist of VLA-4 and a4b7.
N
Cl
NH2
Cl
8
9
9a
9b
9c
Cl
Cl
N
SO2
N
R
Cl
Cl
Cl
Cl
OMe
OMe
O
a
b,c,d
91%
O
Acknowledgements
N
62%
HN
Cl
Cl
NH2
NH2
We wish to thank Dr. J. Fenyk-Melody, Ms. J. Wang,
Ms. S. Zheng, Ms. M. Donnelly, and Ms. Z. Wang for
their assistance in rat pharmacokinetic studies.
10
11
H2N
CO2tBu
Br
e,f
g,h,i
70%
N
N
References and notes
18%
N
Cl
N
Cl
Cl
1. For recent reviews, see: (a) Yang, G. X.; Hagmann, W. K.
Med. Res. Rev. 2003, 23, 369; (b) Tilley, J. W.; Sidduri, A.
Drugs Future 2001, 26, 985; (c) Holland, G. W.; Biediger,
R. J.; Vanderslice, P. In Annual Reports in Medicinal
Chemistry; Doherty, A. M., Ed.; Academic: New York,
2002; Vol. 37, p 65.
Cl
12
13
H
R
N
CO2H
N
SO2
O
Cl
14a: R =
2. For a review, see: Panes, J.; Granger, D. N. Gastroenter-
ology 1998, 114, 1066.
NH
N
Cl
Cl
3. For recent reports of small molecule a4b1 and/or a4b7
antagonists, see: (a) Khandelwal, A.; Narayanan, R.;
Gopalakrishnan, B. Bioorg. Med. Chem. 2003, 11, 4235;
(b) Stasiak, M.; Mehlin, C.; Boni, E.; Vaisar, T.; Little, T.;
Kim, H.-O.; Qabar, M. Bioorg. Med. Chem. Lett. 2003, 11,
3875; (c) Doherty, G. A.; Yang, G. X.; Borges, E.; Tong,
S.; McCauley, E. D.; Treonze, K. M.; Van Riper, G.;
Pacholock, S.; Si, Q.; Koo, G. C.; Shah, K.; Mumford, R.
A.; Hagmann, W. K. Bioorg. Med. Chem. Lett. 2003, 11,
1891; (d) Porter, J. R.; Archibald, S. C.; Brown, J. A.;
Childs, K.; Critchley, D.; Head, J. C.; Parton, T. A.;
Robinson, M. K.; Shock, A.; Taylor, R. J.; Warrellow, G.
J. Bioorg. Med. Chem. Lett. 2003, 13, 805.
Cl
Cl
14b: R =
14
Cl
Scheme 2. Reagents and conditions: (a) 2,6-Cl2–PhCHO, PhNO2,
toluene, reflux; (b) allyl bromide, Cs2CO3, CH3CN, rt, 2 h, 1:1 mixture
of regioisomers (only one is shown); (c)–(g) same as in Scheme 1;
(h) PhSiH3, Pd(PPh3)4, AcOH, CH2Cl2, rt, 4 h; (i) TFA, CH2Cl2.
Table 1. Summary of binding and pharmacokinetic properties10
4. (a) Hagmann, W. K.; Durette, P. L.; Lanza, T.; Kevin, N.
J.; de Laszlo, S. E.; Kopka, I. E.; Young, D.; Magriotis, P.
A.; Li, B.; Lin, L. S.; Yang, G.; Kamenecka, T.; Chang, L.
L.; Wilson, J.; MacCoss, M.; Mills, S. G.; Van Riper, G.;
McCauley, E.; Egger, L. A.; Kidambi, U.; Lyons, K.;
Vincent, S.; Stearns, R.; Colletti, A.; Teffera, J.; Tong, S.;
Fenyk-Melody, J.; Owens, K.; Levorse, D.; Kim, P.;
Schmidt, J. A.; Mumford, R. A. Bioorg. Med. Chem. Lett.
2001, 11, 2709; (b) Lin, L. S.; Lanza, T.; McCauley, E.;
Van Riper, G.; Kidambi, U.; Cao, J.; Egger, L. A.;
Mumford, R. A.; Schmidt, J. A.; MacCoss, M.; Hagmann,
W. K. Bioorg. Med. Chem. Lett. 2002, 12, 133.
Com-
pound
a4b1
Ca2þ/Mg2þ Mn2þ
Clearance F%
(mL/kg/
min)
a4b7
Mn2þ
IC50 (nM) IC50 (nM)
IC50 (nM)
1
0.07
0.27
1.7
0.27
0.31
14
22
97
33
39
24
9
9
2
0.7
8
7
33
3.6
52
21
1.8
22
1695
179
5.5
9a
9b
9c
14a
14b
0.74
0.79
0.34
0.54
0.69
15
20
25
4.3
105
3.2
25
21
24
0.3
1.2
5. See Ref. 3c for details of the binding assays of the
unactivated states of VLA-4, and see Ref. 4 for details of
the binding assays of the Mn2þ activated states.
6. For a review of bioisosteres, see: Chen, X.; Wang, W. In
Annual Reports in Medicinal Chemistry; Doherty, A. M.,
Ed.; Academic: New York, 2003; Vol. 38, p 333.
3.0, 4.3, and 2.9, respectively).11 Two aryl derivatives 9b
and 9c were also studied in the rat. In particular, 9c
emerged as a potent dual a4b1 and a4b7 antagonist with

2334
L. S. Lin et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2331–2334
7. Dunwell, D. W.; Evans, D. J. Med. Chem. 1977, 20,
797.
Rowan, K.; Schwinge, V.; Renzetti, L. M. Bioorg. Med.
Chem. Lett. 2002, 12, 2479.
8. Corey, E. J.; Xu, F.; Noe, M. C. J. Am. Chem. Soc. 1997,
119, 12414.
9. Sidduri, A.; Tilley, J. W.; Lou, J. P.; Chen, L.; Kaplan, G.;
Mennona, F.; Campbell, R.; Guthrie, R.; Huang, T.-N.;
10. Pharmacokinetic studies were performed in male Sprague–
Dawley rats with 1 mpk IV or 2 mpk PO administration of
the appropriate compound.
11. The value of log D was determined by HPLC at pH 7.3.